Superplastic forming (SPF) is a fabrication technique capable of forming large and complex workpieces in one operation. Materials having superplastic characteristics exhibit an enhanced ability to be plastically deformed without rupture, a property known as superplasticity. This property is exhibited, for example, by certain fine-grained metals at a homologous temperature, which is a fraction of the material's melting point.
A typical SPF process involves placing one or more worksheets of a material having superplastic characteristics in a die, heating the worksheets to a temperature within the superplastic range, and superplastically forming the sheet(s) at the SPF temperature. Usually, a differential forming pressure from a gas manifold is used to stretch the worksheet(s) into the desired shape against the die surface(s). One advantage of SPF is that complex shapes can be formed from sheet metal so that the time and expense of milling are eliminated with great cost saving. SPF methods are also usually applicable to single and multisheet fabrication.
For multisheet fabrication, SPF is combined with joining processes to produce sandwich structures from stacks of two or more worksheets. For example, combination of SPF with diffusion bonding (DB) is well documented and has been used in the aerospace industry for many years. Also popular is the combination of SPF/brazing, where a brazing compound is applied where bonding is desired, SPF is carried out, and then the faying surfaces are brazed.
However, not all materials having superplastic characteristics are suitable for traditional joining processes. This applies, in particular, to certain alloy systems, particularly those of aluminum, magnesium, and beryllium, which do not lend themselves easily to DB or brazing. Without being bound to any theory, the main obstacle appears to be the tendency of such metals and their alloys to form tenacious and chemically stable surface oxide layers that interfere with the formation of a metal-to-metal contact between the faying surfaces during welding. Oxide layers are particularly troublesome in the case of aluminum; aluminum oxide is denser than and has a melting point that is twice that of pure aluminum. Accordingly, prior to welding, steps for cleaning off the oxide layer are usually needed, for example with a wire brush and/or acetone, so that the material underneath the oxide layer can be exposed.
Another disadvantage of DB relates to instances where one or more of the sheets do not react well to heat treatments, such as the protracted heating typical of DB. Moreover, DB may not be applicable in instances where one or more of the sheets to be joined are heterogeneous in nature, such as a sheet having a first side of a first alloy characterized by a melting temperature of 500° C., and a second side of a second alloy whose melting temperature is 700° C. instead. A temperature sufficient for carrying out DB on the first side of the sheet is likely too low for the second side; conversely, a temperature sufficient for performing DB on the second side may induce undesirable melting of the first side. Additionally, in processes including both SPF and DB, the majority of the cycle time is taken by the DB. Typically, such processes are carried out either in a die, tying up a valuable asset, or in a dedicated furnace, which means additional equipment is needed.